1. Field of the Invention
The invention relates to a light emitting diode including a light emitting element and a phosphor. More particularly, the invention is concerned with a light emitting diode including a light emitting element and a phosphor in which light of a predetermined color emitted from the light emitting element is mixed with photoluminescence given off from the phosphor to provide light having a mixed color which is then radiated to the outside of the light emitting diode. In this specification, an LED chip per se is referred to as “light emitting element,” and the whole system including an LED chip-mounted package resin or lens system or other optical system is referred to as “light emitting diode.”
2. Related Art
Light emitting elements, which are generally used in light emitting diodes, include inorganic light emitting elements, laser diodes, inorganic thick film electroluminescence sheets, and inorganic thin film electroluminescence components. Among others, inorganic light emitting elements have outstanding features including long service life, space saving, good impact resistance, and narrow-band emission spectrum.
A large number of emission colors, particularly a large number of emission colors with broad-band emission spectrum cannot be realized by light emission inherent in active semiconductor materials in inorganic light emitting elements, or can be realized only with low efficiency. In particular, this is true of the provision of white light emission.
Emission colors which cannot be realized by semiconductors have hitherto been provided by a wavelength conversion technique. The wavelength conversion technique is essentially based on the following principle. Specifically, at least one phosphor is placed on a light emitting element, and the phosphor absorbs light emitted from the light emitting element and gives off light with a wavelength different from the absorbed light. In other words, the phosphor absorbs light emitted from the light emitting element and then radiates photoluminescence with a different emission color.
A light emitting diode, which emits light based on the above principle, is described in Japanese Patent No. 2947344. This light emitting diode is shown in FIG. 1. As can be seen from FIG. 1, the light emitting diode includes a pair of lead frames 113, 114. A cup part 116, which functions as a reflector, is provided in the lead frame 113. A blue light emitting element 102 is fixed to the cup part 116 with the aid of a phosphor-containing adhesive 110. The blue light emitting element 102 is provided with a pair of electrodes 102a, 102b. The electrode 102a is connected to the corresponding lead frame 113 through a bonding wire 105, and the electrode 102b is connected to the corresponding lead frame 114 through a bonding wire 106. The whole assembly has been sealed with a light transparent resin 108.
In this light emitting diode, blue light emitted from the blue light emitting element 102 is partially or entirely subjected to wavelength conversion by the phosphor. As a result, light with a color different from blue light is radiated. For example, when a yellow phosphor is used, yellow light obtained by the wavelength conversion is mixed with blue light not subjected to wavelength conversion to provide a mixed light which is then radiated to the outside of the light emitting diode. Therefore, this mixed light is theoretically seen as white light as viewed from the outside of the light emitting diode.
A reflection light emitting diode may be mentioned as another conventional light emitting diode using a combination of a light emitting element with a phosphor. This reflection light emitting diode is shown in FIGS. 2 and 3.
In this light emitting diode, as shown in FIG. 2, a light emitting body 120 is used in which a light emitting element 102 is mounted on the bottom of a concave 121 and the concave is filled with a phosphor-mixed resin 122. Specifically, as shown in FIG. 3, the blue light emitting element 102 is mounted on the bottom of the concave 121 so as for the light emitting surface of the blue light emitting element 102 to face downward, and the concave 121 with the blue light emitting element 102 mounted therein is filled with the phosphor-mixed resin 122. This light emitting body 120 is used as a light source. A reflector 124 in the form of paraboloid of revolution of which the focal point is the light emitting element 102 is disposed opposite to the light source, and the whole assembly is sealed with a transparent epoxy resin 125.
The light emitting diode shown in FIG. 1 and the reflection light emitting diode shown in FIGS. 2 and 3, however, have the following problems.
In the case of the light emitting diode shown in FIG. 1, the quantity of light emitted from the upper surface of the blue light emitting element is larger than the quantity of blue light emitted from the side or lower part of the blue light emitting element. On the other hand, since the layer thickness of the phosphor is relatively small, the quantity of light absorbed in the phosphor layer is small. Due to these facts, the quantity of yellow light emitted is unsatisfactory. For this reason, when the upside of the blue light emitting diode is viewed from the outside of the diode, the emission color perceived by the viewer is not a contemplated color, that is, is not white. Specifically, the emission color of the center portion is perceived by the viewer to be somewhat bluish, and the emission color around the center portion is perceived by the viewer to be somewhat yellowish. Thus, the light emitting diode shown in FIG. 1 is disadvantageous in that a desired color such as white color cannot be uniformly emitted from the whole light radiating surface of the light emitting diode.
On the other hand, in the reflection light emitting diode 123 shown in FIGS. 2 and 3, the whole concave 121 emits light. Therefore, the size of the light source is large, and this large light source size poses various problems. Specifically, when white light provided by mixing blue light with fluorescence has been applied downward from this light source, the white light should be reflected substantially perpendicularly from the reflector 124 and radiated upward. However, this cannot be achieved by the following problems: (i) the size of the concave 121 constituting the light source is so large that light reflected from a portion around the center of the reflector 124 is blocked by the concave 121 and cannot be radiated outside the light emitting diode; and (ii) since the diameter of the light source is large, the light beam is spread and consequently is disadvantageously reflected obliquely from the reflector 124 and cannot be radiated in a substantially perpendicular direction. Further, the reflection light emitting diode shown in FIGS. 2 and 3 has an additional problem (iii) that, because of the large original light source, white light, which has been radiated outside the light emitting diode, cannot be focused to a small size by a focusing optical system.
Additionally, coating of the phosphor-mixed resin 126 onto only a portion around the light emitting element 102 to provide white light as shown in FIG. 4 is considered. In this method, however, the concentration of the phosphor in the resin should be increased, and the increased phosphor concentration causes light emitted from the light emitting element 102 to be absorbed and attenuated in the phosphor-mixed resin 126. This disadvantageously poses a problem of low fluorescence excitation efficiency.